1. Field of the Invention
The present invention relates to devices for synchronising an output signal with an input signal, and in particular to devices which use interpolation or mixing of two reference sources to generate the output.
2. The Prior Art
Phase locked loops (PLLs) are well known electronic devices and they function to provide an output oscillating signal which is phase locked with an incoming signal. In most PLL architectures, the output signal is generated by a voltage controlled oscillator (VCO) and this output is compared by a phase detector with the input signal. This produces an error signal representing the phase difference between the input and the output and this error is applied, typically via an appropriate filter, to alter the control voltage of the VCO. The oscillating frequency of the VCO is therefore altered until such time as the output is in phase with the input at which time no further error signal is generated and no further changes are made to the control voltage input to the VCO.
PLLs are typically used where it is desired to generate a clean clock signal synchronised with an incoming signal. One particular application of PLLs is in data transmission systems, for instance communication devices where the transmitted signal is modulated based on a signal having a particular frequency and it is important to recover a corresponding clock signal at the receiver in order to demodulate the received signal.
In a typical communication system there may be many PLLs in close proximity. For instance there may be PLLs both in the transmit side of the system and in the receive side, and also in a multi-channel system there may be a PLL for each of the channels. If such arrangements are implemented on a single chip problems can occur due to interference between VCOs implemented in such close proximity. Also as data rates increase, the power consumption of the VCO required to generate a satisfactorily low-noise signal increases and this is compounded if more than one VCO is implemented in any particular device.
It is further known that clock signals may be used in a number of parts of any specific electronic circuitry. In a physical implementation therefore clock signals are carried around a chip or circuit board arrangement by suitable conductive means so as to be transferred from the clock generator to the part of the circuitry dependent on the clock.